1. Field of the Invention
This invention relates to a photo-sensitive and heat-sensitive composition which is devised to be fixed when irradiated with visible light and heated, and to generate color when irradiated with ultraviolet rays to thereby produce a high quality image, and a recording element comprising a support and a layer of said composition superposed thereon.
2. Description of the Prior Art
A considerably large number of photo-sensitive compositions, photo-sensitive and heat-sensitive compositions and image-forming recording elements using said compositions have been widely utilized or proposed up to now. As typical examples there can be enumerated, for instance, diazo copying materials using alkali media as developers, photographic films using silver salts, etc. However, the above-mentioned photo-sensitive compositions and recording elements are defective in the fact that developers and fixers must be used in order to obtain an image.
As an attempt to eliminate the foregoing defect, public attention has been attracted to dry photochemical photo-sensitive compositions and recording elements capable of forming images only with light as described in Japanese Patent Publication Nos. 19161/1968, 40150/1970, 17855/1972, Japanese Laid Open Patent Application No. 12879/1972, etc. For instance, the photo-sensitive composition as described in Japanese Patent Publication No. 19161/1968 comprises (a) a photooxidant such as hexaarylbiimidazole, tetraarylhydrazine or the like, (b) a color-generator such as leuco-pigment, hydrazone or the like and (c) a binary system redox couple comprising an oxidant component such as quinone, ketone or the like and a reductant component such as alcohol, ester or the like or a one component system redox couple such as 1,4-bis(2-methoxyethyl)anthraquinone or the like. The said photo-sensitive composition generates color by means of light of specific wavelength, such as ultraviolet rays. The color generating reaction is performed by the action of a photooxidant and a color-generator, and the originally generated color is deactivated permanently, that is, fixed by light of a different wavelength, such as visible light. The fixing reaction is performed by the action of said redox couple. The photo-sensitive composition of this type, accordingly, can obtain both positive images and negative images by selection of the sequence of irradiation of ultraviolet rays and visible light, but is defective in that the fixing sensitivity (non-color-forming sensitivity) is still low.
Japanese Patent Publication Nos. 139722/1975, 139723/1975, 139724/1975, etc. each disclose an image-forming recording element comprising a support and a photo-sensitive and heat-sensitive layer superposed thereon. The photo-sensitive and heat-sensitive composition used herein is mainly composed of (a) a cobalt(III) complex and (b) a photoreductant such as quinone, diazonium salt, disulfide or the like. In the case of the composition of this sort an image is formed in such a manner that the photoreductant generates a reducing agent when irradiated with visible light, and this agent reduces and discolors the cobalt(III) complex when heated (at the same time the cobalt(III) complex generates a basic substance such as ammonia or the like). Moreover, Japanese Patent Publication No. 139724/1975 proposes to accelerate the reduction of the cobalt(III) complex as well as promote the developed color density by the addition of a chelating agent such as nitroso-arol, dithiooxyamide or the like. At any rate, the use of such a chelating agent-containing photo-sensitive and heat-sensitive composition, on the one hand, is advantageous in that a high sensitivity image can be obtained as compared with diazo copying materials, aided by the aid of the oxidation-reduction reaction between the cobalt(III) complex and the photoreductant. On the other hand this type of composition is defective in the following points, namely, (1) since the coloration of the composition is effected by the action of generated chelate (II) or (III) compounds there can be obtained only a negative image, and (2) the obtained image undergoes severe thermal fogging (in the heating step, the periphery of the exposed area also generates color) and the like.